Positron emission tomography (PET) is a rapidly evolving imaging modality that plays a key role in drug development, disease diagnosis, patient screening, and treatment monitoring. Despite exciting progress, imaging of many additional targets is desired, and development of new PET agents is constrained by available labeling methods. In particular, due to the short half-lives of fluorine-18 and carbon-11 nuclides, radiosynthesis must be fast and efficient. Mild radiosynthesis conditions are also preferable which could allow late-stage radiolabeling with most functionalities already installed, leading to higher step efficiency. To meet the greatly increased demand for mild and efficient reactions, photoactivated radiosyntheses have emerged with unique features: the photoactivated intermediates can be highly reactive, and the photoactivation process can be performed at room temperature with low-toxicity reagents. This chapter focuses on the recent progress of photoactivated radiosyntheses with fluorine-18 and carbon-11 nuclides and provides an overview of the application of photoactivated processes in radiochemistry.